1. Field of the Invention
The present invention relates to interference cancellation in multiple access communications systems which permit multiple simultaneous users. Well-known applications include cellular telephone systems, Personal Communications Services (PCS), wireless Local Area Networks (LAN), etc. These systems must rapidly identify and separate the transmissions of each user from the interference or “noise” that is represented by all the other simultaneous users. It will be appreciated that the task of identifying and separating all simultaneous transmissions—in real time—is enormously complex. The more simultaneous users, the harder this task becomes. The result is a significant bottleneck in the receiving and information identification functions of these systems.
Current systems have attempted to minimize the complexity of the identification task by simply limiting the number of simultaneous users, well below what is theoretically possible. Limiting the number of simultaneous users places an artificial limitation on the bandwidth capacity used at any one time—that is, such current systems do not make full use of the bandwidth available to them. Such systems are thus downsized to accommodate the bottleneck.
In the example of cellular telephony, permitting a larger number of callers would increase the noise levels above the system's capacity to identify and eliminate it. The system would not be able to separate each user's transmission from the noise that is the other users' transmissions. Users may encounter dropped or disconnected calls or transmissions (as additional users access the system), or the system may become temporarily “unavailable” to users wanting to place calls or transmit data. Moreover, as user demand continues to increase, service providers face intense pressure to reliably service more users simultaneously. There is an urgent need for an improved receiver function which can accommodate greater numbers of simultaneous users, while performing the information identification functions necessary to separate the transmissions.
Complex mathematical algorithms (referred to herein as “receiver algorithms”) have been developed which could assist in the task of signal-to-noise enhancement. They are not presently used in communications systems due to limited ability to generate the large quantities of data needed. These receiver algorithms operate by estimating interference and/or by estimating signal parameters. Each signal must be rapidly and repeatedly correlated against known or estimated information (“hypotheses”). These hypotheses may include, for example, the unique codes assigned by the system to each transmission or portions thereof. A computer processor equipped to generate the required number of simultaneous calculations would be too large, too heavy and too costly to be of practical utility. Consider, for example, that in cellular telephone systems, the equipment for rapidly identifying and separating the transmissions is located on or near each receiving tower.
In a preferred embodiment, the present invention employs multichannel optical correlators to great advantage to allow commercial implementation of receiver algorithms in multiuser systems. The multichannel optical correlators conduct parallel (simultaneous) processing of the signal against huge numbers of hypotheses to generate data (“correlations”) useful in identifying each transmission. This parallelism makes optical correlators uniquely and ideally suited for processing complex data streams and simultaneously conducting billions of calculations to generate data (correlations) with respect to the individual transmissions in that stream. The data generated by use of the optical correlator is fed to one or more receiver algorithms, which then identify, sort and separate the transmissions of the various simultaneous users. In this manner, multiuser communications systems will be able to maximize bandwidth usage to accommodate significantly larger numbers of simultaneous users than is possible today.
Additional technical background information and definitions are found following the Detailed Description of this disclosure, identified as “Technical Background.” This section also provides further identification for some of the acronyms used in the description in accordance with the present invention.
2. Description of Related Art
A variety of approaches have been suggested to remove unwanted interference from desired signals in communications systems. As described above, the complexities of this task can increase exponentially with increasing numbers of simultaneous users and increasing length of transmission per user.
A large majority of these prior approaches require a bank of matched filters and/or correlators. Purely digital systems have been employed to perform correlation/matched filter function, as, for example, in U.S. Pat. No. 5,903,550 (Spock), which describes a digital system to perform demodulation of a CDMA signal with many time offsets and demodulating different users sequentially in time. Disclosed in the Spock patent without detail is a “parallel despread” and a “parallel chip integration” that forms a parallel correlator. This correlator (which is not an optical correlator) works on a set of stored samples of the incoming signal.
Analog correlators are disclosed in the prior art. U.S. Pat. No. 4,267,580 (Bond et al.), U.S. Pat. No. 4,813,006 (Bums et al.), U.S. Pat. No. 5,126,682 (Weinberg et al.) and U.S. Pat. No. 5,276,705 (Higgins) are all examples of analog correlators based on charge coupled device (CCD) technology. While these references do not specifically identify the communications applications taught herein, the use of CCD correlators is well known. U.S. Pat. No. 3,937,942 (Bromley) describes the use of a certain class of photosensors known as Time Delay and Integrate (TDI) sensors for correlating an unknown incoming signal with one or more of a great number of stored known reference signals. The devices disclosed in these patents are suitable for use as components in the improved receivers in accordance with the present invention, and the patents listed in this paragraph are incorporated herein by reference.
Various optical systems, including optical correlators, have been used previously in other contexts or for processing received signals for various other purposes. Two-dimensional (2-D) optical correlators are known to the art, particularly in pattern matching applications, such as fingerprint identification or military targeting applications. The utility of such correlators in the present invention is doubtful. Two-dimensional correlators correlate a single 2-D object, such as an image, against a single 2-D hypothesis, generally testing all possible 2-D (x and y dimensions) offsets of the hypothesis. In general, they perform a single 2-D integration. By contrast, the optical correlators most useful in the present invention are those that can correlate a single, one-dimensional (1-D) object against a large set of 1-D hypotheses, producing multiple results. That is, the bank of 1-D correlators performs a set of independent 1-D integrations.
Optical correlator architectures suitable for use in accordance with the present invention possess the quality of using the two available dimensions separately as individual codes in one dimension and time offsets in the other. In addition, the correlator preferably has the facility for introducing a set of vectors of hypotheses values into the optical beam in parallel, and providing as output a time-varying vector representing the results of the correlation of each of the hypothesis at one time offset where the time offsets can vary in time.
Suitable optical correlators of various architectures are known to the art. U.S. Pat. No. 4,225,938 (Turpin) describes several time-integrating acousto-optic correlator processors for performing correlation and other functions. Of particular interest is the description of time-integrating correlation of columns 3–4. U.S. Pat. No. 4,833,637 (Casasent et al.) describes an acousto-optic multichannel space integrating correlator. U.S. Pat. No. 4,468,093 (Brown) describes the use of a space/time integrating optical processor for performing time versus frequency cross-correlation. U.S. Pat. No. 4,620,293 (Schlunt et al.) illustrates a type of optical correlator for performing a linear multiplication operation and U.S. Pat. No. 4,843,587 (Schlunt et al.) illustrates an optical correlator for performing matrix multiplication. The optical correlators disclosed in these patents are suitable for use in the improved receivers in accordance with the present invention, and the patents listed in this paragraph are incorporated herein by reference.
However, none of these references teach or suggest the use of optical correlation technology to generate the matrices of data needed for receiver algorithms. The present invention thus makes a marked advance in communications technology by providing a highly efficient method and apparatus for generating such data matrices for use in various receiver algorithms.